I read Girl Genius, Erfworld, OoTS and xkcd as well! I've at least checked out almost all the rest mentioned in this thread as well, liking some more then other. I recently picked up a new one (to me, it's not new at all) names Unshelved. It's very funny, I read about a year's worth of back comic in the past 2 days. While almost all can stand alone, it does follow a time line so starting at the beginning makes the inside jokes click.

1/0 - Somebody pointed this out to me recently. It's no longer updated, but pretty good. Lots of discussions about the 4th wall.The rest I read are either already said or not good enough.Or I've forgotten them.